In a mesa semiconductor device by the prior art, as for methods depositing a metal film wiring pattern for an electrode pad on a surface of the semiconductor device, the following methods are well-known; a method depositing a metal film on a desired area by an electroplating process, and a method in which a metal film is deposited on a whole surface of a wafer by a sputtering process or a vacuum evaporation process, and successively retained on a predetermined area by patterning in a photolithographic process.
For example, as shown in FIG. 3A, a recess is formed at an area for separating a wafer into pellets in which a p-n junction is formed by p-type layer 22 laminated on an n-type semiconductor substrate 21, and an insulating film 23 such as a passivation film or the like is formed by depositing SiO2 film or sintering glass paste coated on the recess surface, and thereafter two electrodes 24, 25 are formed by depositing films on a surface of the semiconductor layer 22 and the reverse side surface of the semiconductor substrate 21 by an electroplating process. In this case, an electrode 24 of a face side is provided on not only the surface of the semiconductor layer 22 but also the insulating film 23 with a hang of an electroplated metal, whereby a problem of increasing a strayed capacitance or an electrical static damage on a side of mesa structure occurs more easily. In order to solve these kinds of problems, it is also suggested to form an exposed area by providing a photo resist film on a surface of the semiconductor layer 22 and form a metal film only on the exposed area by an electroplating method (cf. Japanese Unexamined Patent Publication No.HEI1-232719).
Another method is also adopted, such as, as shown in FIG. 3B, depositing metal films 24a and 25 provided on the whole surface of a face and a back of a semiconductor wafer in which a p-n junction is made in a same manner described above, and a part of the metal film 24a and the semiconductor layer 22 are etched with coating a photo resist film not shown, thereby a recess is formed at an area, where a wafer is divided into pellets, to form a mesa structure and an electrode 24, thereafter an insulating film 23 is formed by sintering glass paste or the like coated on the recess surface (cf. Japanese Unexamined Patent Publication No.SHO56-58232).
As described above, in the case of forming the electrode on the surface of the semiconductor wafer which has the recess and the insulating film provided on the recess, when the electrode being deposited on the semiconductor layer surface, a hang of the metal film of the electrode is formed over the insulating film which results in an increase of a strayed capacitance or an electrical static damage.
On the contrary, in the case of forming the electrode on only a desired area by the electroplating method by forming a resist film on the whole surface of the wafer and then exposing only a desired surface of the semiconductor layer except the recess, as the wafer which has recess is liable to warp and the recess makes a surface of the wafer more uneven, so it becomes difficult to make a precise patterning and the electrode accurately on the predetermined area. In the method of depositing a metal film (an electrode) on a desired area by patterning a resist film after forming the recess and the insulating film on the recess, the problem described above occurs similarly in a case of not only the electroplating process but also a sputtering process or a vacuum evaporation.
And in the above case, of forming the recess and the insulating film after depositing the metal film on the whole surface of the semiconductor wafer, when making a thick insulating film especially for a high voltage use, as it is necessary to coat glass paste and sinter it at a temperature from 600° C. to 800° C., so there becomes a problem that normally used material like Al for the electrode can not be applied, because a material of electrode needs to stand a sintering temperature.
Moreover, a metal film by an electroplating has a problem like a deterioration in electric characteristics, for example VF (forward direction voltage drop), caused by a less uniformity of the thickness of metal film within a wafer surface which is induced by a variation in the growing rate of plating, between at a center and an edge of the wafer by movement of plating solution by stirring, depending on a distance between a part of the wafer and a plating electrode which is contacted at a center of the wafer by a point contact, and depending on a concentration of current to an irregular point by a pattern defect in a back surface of the wafer.